A cooling circuit through which coolant for cooling the engine flows incorporates a thermostat that is opened or closed according to the temperature of the coolant. The thermostat opens the valve and allows the coolant to flow into a radiator when the temperature of the coolant is higher than or equal to a valve opening temperature. In relation to such a thermostat, Patent Document 1 discloses a technique of determining whether the thermostat is stuck open by comparing a coolant temperature detected by a coolant temperature sensor with an estimated temperature of the coolant temperature when warm-up of the engine is completed after the engine is started when it is cold.